This invention relates to a direct electrostatic printing device and more particularly to a developer or toner delivery system for presenting developer or toner to an electronically addressable printhead utilized for depositing developer in image configuration on plain paper or intermediate substrates.
Of the various electrostatic printing techniques, the most familiar is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A less familiar form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the aforementioned xerographic form, in that, the toner or developing material is deposited directly onto a plain (i.e. not specially treated) substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sep. 5, 1972 to Gerald L. Pressman et al.
Pressman et al disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charge particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al device the supply of the toner to the control member is not uniformly effected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fujii et al discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of charged particles and to record a visible image by the charged particles directly on an image receiving member. Specifically disclosed therein is an improved device for supplying the charged particles to a control electrode that has allegedly made high-speed and stable recording possible. The improvement in Fujii et al lies in that the charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. Fujii et al purports to obviate the problems noted above with respect to Pressman et al. Thus, Fujii et al alleges that their device makes it possible to sufficiently supply the charged particles to the control electrode without scattering them.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a recording electrode and a signal source connected thereto for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. An A.C. and a D.C. source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller.
Each of the foregoing patents makes use of individually addressable holes to gate toner pixelwise to an imaging member. Key to each of these schemes, as well as the feature which makes each of these printing methods unique, is the method by which toner is presented to the printhead. For example, the '855 patent employs jumping development. There are inherent problems with this (AC jumping) scheme. First, the spacing between the donor roll surface and the print head is very critical. Experiments in the laboratory indicate that &lt;0.001" tolerance is required.
Another problem arises in high speed printing. The donor roll must have a uniform layer of toner on it and requires fast reloading. The tight DEP requirements on a percentage of wrong sign toner requires a very controllable charge distribution on the toner delivered to the printhead. A two component system such as disclosed in U.S. Pat. No. 4,814,796 granted to Fred W. Schmidlin on Mar. 21, 1989 which loads a donor roll helps with the problem of toner charge distribution and other problems peculiar to single component systems but the problem of tolerances between the donor roll and printhead are not solved thereby. Other problems are introduced especially for color two component materials at reasonable print speeds. Since the donor must be reloaded at 100% area coverage for any print area coverage such a system is always running in a stress mode with respect to admix. Also, undeveloped toner must get distributed back onto the carrier beads, a problem which is exacerbated with color developers.